1. Field of the Invention
The present invention relates to the measurement of the size of cells or pores, such as plastic foam cell size, and more particularly, to the measurement of plastic foam cell size using a wax impression visualization technique.
2. Description of Background Art
Cells in plastic foams are typically in the form of voids or gas filled cavities formed throughout the foam. These cells are geometrically defined by the foam's composition having an overall form similar to that of a honeycomb. Cell size measurements for plastic foams such as polyurethanes, for example, are useful as a development tool to help application scientists produce foams with more desirable physical properties, in the quality control of foam manufacture and, in some cases, as an aid in setting foam specifications for sale or purchase. Cell size measurements include whole cell dimensions (maximum and minimum diameter), cell window dimensions, cell strut lengths and widths, cell size distributions, and the number count of cells falling along a straight line, e.g. cells per inch. Cell struts are the relatively thick sections of solid material that act as a foam's backbone. Cell windows are the thin, usually transparent, sections of solid material between cell struts in closed cell foams and are small openings between struts in open cell foams. There are typically 8-14 windows on the surface of a single cell. The physical properties of foams are generally believed to depend upon the structure of both the polymer network, i.e. the chemistry and morphology of the foam and the cells formed at the end of the expansion process.
One of the most common quantitative measurements of plastic foams is that of cell size. This, however, is not without some ambiguity with respect to the concept of the cell, particularly in open cell foams. Close visual inspection of a foamed polymer reinforces the difficulty inherent in defining a cell. The measurement problem is further complicated by the limited number of potentially valid linear measurements available to define a cell. These range from a direct linear measure of the maximum dimension of a cell face or edge, to a calculated size from an average of cord lengths, to a more detailed measurement of intercepts as these terms are understood by those skilled in the plastic foam art.
Some of the existing methods or techniques for the measurement of plastic foams include: (1) manual measurement of cell dimensions using optical microscopy; (2) manual counting of the number of cells along a fixed length employing optical microscopy; (3) visual comparison with an accepted standard foam utilizing optical microscopy; (4) visual comparison with an optical grid of known dimensions using microscope; (5) measurement as above after enhancing the foams surface for visualization by coloring, dying, or dusting; (6) measurement after visualization by projecting the image of a thin slice of foam on a screen; (7) indirect diameter calculations via strut length measurements by optical microscope; (8) measurement with the aid of a scanning electron microscopy (SEM) after appropriate sample preparation (gold coating); and (9) measurement with an optical microscope after enhancement by embedding the foam sample in a plastic resin, curing, cutting, and polishing the specimen.
Difficulties in the measurement of cell size in plastic foams are that the simpler or faster methods such as methods 1-6 listed above, often lead to inaccurate results and, moreover, are generally not reproducible because there is a high level of operator interpretation required. Optical microscopic analysis of foams also has the disadvantage that it is difficult to resolve cells in a three-dimensional foam lattice due to light scattering and depth of field limitations preventing a sharp focus of the cells. Furthermore, the inability to adequately measure cells with missing or distorted cell features like broken struts is also a problem. Finally, the inability to distinguish between full cells and the cell windows which make up the cells also contribute to the inaccurate results of optical microscopic analysis of cellular foams.
The method of indirect diameter calculation via strut length measurements by optical microscopy (method 7) is based on a mathematical model of the cell structure and can give a reasonable representation of cell diameters for well ordered foam samples, however, mathematical models are not well suited for foams which are not well ordered or where there is a fairly wide distribution of cell sizes.
More accurate and reproducible methods such as the use of scanning electron microscopy and enhancement by embedding the foam sample in a plastic resin, curing, cutting, and polishing the specimen (methods 8 and 9) require expensive instrumentation (SEM) or take an extended amount of time for sample analysis, typically greater than one operator hour per sample. Further, such methods, while generally more accurate than other methods, still do not necessarily provide for highly accurate measurements of foam cell components. Such methods are useful for periodic or occasional measurements for research and development purposes but are generally not suited for large numbers of samples.
Accordingly, it is desirable in the art of foam cell testing and analysis to provide a measurement technique for measuring cell size of a plastic foam which is accurate, reproducible, inexpensive, and requires a relatively small amount of time per sample.